Optical fibers were first deployed in telecommunications primarily as a medium for transmitting information across long distances. In that role, optical fibers carried voice and data traffic between switching points separated by distances of several or more kilometers. In many networks, however, electrical conductors continued to be used to connect the switching points with the sender and receiver of the communications. Therefore, as an example, data transmitted from a business to a home would first pass through a network of electrical conductors between the business location and a first switching point, then pass through optical fibers across a long distance to a second switching point, and finally pass through another network of electrical conductors to arrive at the home.
Increasingly, optical fibers are replacing electrical conductors within the short distances between switching points and the ultimate sender and recipient of communications. This portion of the telecommunications network is often referred to as the “last mile” of the telecommunications network. Providing a complete optical-fiber path between a source of data and a recipient will enable more use of high-bandwidth applications and is part of a movement known as Fiber to the Home (FTTH), Fiber to the Premises (FTTH), and other generally FTTx.
Besides providing optical fiber to individual residences, FTTx contemplates the installation of optical fiber to multiple dwelling units (MDUs). MDUs are buildings in which a large number of people require separate access to telecommunications through the optical network. In particular, MDUs may include apartment buildings, condominiums, townhouses, dormitories, and hotels. Although not residences, MDUs may also include office buildings, schools, and factories.
MDUs can present a substantially different environment for installing optical fibers and cables than a single-family residence, or single dwelling unit (SDU). With an SDU, it is often most practical to span the final network segment (to the residence) with a single fiber drop. On the other hand, MDUs may have individual residences that are in close proximity to each other. Consequently, although single fiber drops can be used, it will be less time consuming to pull multiple drop cables in a single operation.
Indoor optical cables are well known. Among other criteria, these cables are generally requested to have sufficient flexibility and tensile strength and be flame retardant. Several standards of the National Electric Code (NEC) and other organizations specify performance requirements. Indoor optical cables are often rated for either riser (vertical) or plenum (horizontal) applications.
The most basic type of indoor optical cable is called “simplex” cable. By “optical cable,” it is meant a cable having at least one optical fiber, which fiber comprises an optical transmissive core generally made of glass and a polymeric coating, usually made of a resin that is typically curable with ultraviolet or infrared light. In the simplex design, a single optical fiber is directly coated with a protective layer of polymeric extrudable material, known as being tight-buffered. The tight-buffered optical fiber is usually surrounded with a reinforcing layer, typically based on aramid yarn, and then surrounded by a flame-retardant jacket. With this form, the simplex cable can easily be bent and routed through small spaces in a building. Moreover, tight-buffered cables may be directly connectorized. U.S. Pat. No. 7,403,687 describes several exemplary types of simplex cables.
While potentially satisfactory for SDUs, simplex cables can be insufficient for wiring an MDU. For example, simplex cables contain only one fiber, and pulling individual cables for each resident within an MDU can be time-consuming. Also, depending on the type of installation and the size of the cables, many simplex cables within an MDU can result in an unsightly installation.
Other types of indoor optical cables contain multiple tight-buffered optical fibers. Premises distribution cables, for example, contain several tight-buffered fibers bundled under the same jacket with strength members to stiffen the cable. These cables are small in size and used for short runs within building conduits. But because the fibers are not individually reinforced or robust, accessing the cable within the field can be difficult without damaging the fibers. They require a “breakout box” or junction box to be terminated, which can restrict their utility.
Premises breakout cables, or fan-out cables, are more robust than distribution cables. Breakout cables generally comprise simplex cables bundled together. Each fiber is reinforced, so breakout cables allow for quick termination to connectors without the need for a breakout box or junction box. Nonetheless, accessing the fibers within a conventional breakout cable can be challenging, especially if midspan access is desired.
FIGS. 1 and 2 depict two examples of conventional breakout cables. FIG. 1 illustrates a breakout cable 100 having two sub-units 102 each comprising a 900 μm tight-buffered optical fiber 104, a dielectric strength member 106 around the optical fiber 104, and flame-retardant jacket 108. Cable 100 includes an outer jacket 110 to encase and protect the sub-units 102 and ripcord 112 to make opening the jacket 110 easier. Although not shown, sub-units 102 are often S-Z stranded and held together with binder threads. FIG. 2 shows a breakout cable 200 that is substantially the same as cable 100 but with more sub-units 102. In this embodiment of FIG. 2, subunits 102 are arranged in two layers around a dielectric central member 202. Binder threads not shown typically hold sub-units 102 in place.
Removing the jacket in a conventional breakout cable, such as those shown in FIGS. 1 and 2, is burdensome because many steps are required. First, the jacket must be ring-cut at two points in close proximity. Typically, the ring cuts are between 7 cm and 15 cm apart, depending on the toughness of the jacket and the strength of the ripcord. Second, a longitudinal cut is extended from one ring-cut to the other. During each of these cuts, great care must be taken to avoid cutting too deeply, so as to avoid damaging the underlying sub-units. Third, after removing the cut section of the jacket, the ripcord needs to be located, severed, gripped with pliers at one end, and pulled to tear the jacket longitudinally to whatever length is desired.
Some options have been disclosed for easing access to optical fibers within a cable. For example, U.S. Pat. No. 6,603,908 discloses a buffer tube that purports to allow easy access to signal carrying fibers disposed within the buffer tube. In one embodiment of the '908 patent, grooves or indents are continuously or sequentially incorporated into the buffer tubes as stress risers. These stress risers reduce the energy needed to tear into the buffer tube with a ripcord or eliminate the need for a ripcord altogether. The stress risers can be formed on an inner or outer surface of the buffer tube and may be such that bending or twisting will cause seams in the tube to split. Even abrading the exterior of the buffer tube with a material such as sandpaper may be a sufficient approach to weaken and open the buffer tube. The '908 patent appears to only consider providing easier access to optical fibers within buffer tubes, the buffer tubes being enclosed in larger cables and protected by an outer jacket and other conventional materials.
U.S. Pat. No. 7,123,801 relates to optical fiber cables having a central strength member encircled by a circumferentially continuous jacket or sheath. The sheath has at least one longitudinally extending chamber or duct that receives one or more optical fibers, the jacket being manually separable from the strength member. The slots or ducts can extend helically around the strength member but, preferably, the slots or ducts extend around the strength member in S-Z, or alternating lay, fashion. The outer surface of the jacket bears indicia that will identify the location of the slots or ducts in the jacket. The indicia can be grooves in the outer surface, which can be lines of weakening for removing the jacket and/or guides for a cutting tool for cutting the jacket. The manufacturing of such a cable involving ducts for optical fibers can be complicated and expensive.
The abstract of JP1987-049310 relates to a tube for optical fibers having two continuous grooves with V-shaped sections. For instance, the inner diameter of a plastic tube is 3.3 mm, and its outer diameter is 5 mm. Two V-shaped grooves are formed on the outer surface of the tube in parallel with the axis of the tube, and the depth of the grooves is 0.5 mm. Consequently, the tube can be easily cut off by using the V-shaped grooves. The optical fibers appear to be in ribbon format, and the document does not indicate any type of stranding or binding for the fibers.
U.S. Pat. No. 7,054,531 discloses an optical fiber premises cable with a plurality of unit cables. Each unit cable contains a plurality of tight-buffered optical fibers surrounded by a thin polymeric non-load bearing tube. According to the '531 patent, the unit cables aid in segregating and identifying individual tight-buffered optical fibers. The unit jackets are relatively thin and intended to be weak and easily severed. They are not intended to provide physical protection for the tight-buffered optical fibers. Instead, protection is provided by strength members, such as aramid fibers, that are located within the outer cable jacket of the unitized cable rather than directly around the tight-buffered optical fibers within the unit jackets.
U.S. Pat. No. 4,828,352 relates to a fiber optic cable containing a core of S-Z stranded optical fibers composed of first and second alternatingly repeating and essentially equal first and second sections. The optical fiber layers are circumscribed by a plastic sheath in which there is a plurality of grooves in the jacket marking the general vicinity where the first section of the S-Z stranding joins the second section. The grooves are provided perpendicular to the axis of the cable.
Another option for easing the ability to access optical fibers within an optical cable is to eliminate the jacket entirely. This product is essentially an unjacketed breakout cable. In general, an unjacketed breakout cable (or fan-out cable) contains individual simplex sub-units stranded or bundled together and held in place with binder threads. The individual sub-units can be sized to meet the application. Problems with accessing the optical fibers are avoided because there is no jacket to remove.
An unjacketed breakout cable has several disadvantages, however. For instance, during manufacturing, the process for binding the sub-units together is typically very slow relative to standard jacketing operations. During installation, the binders can impede pulling operations. Each wrap of the binder presents an opportunity for a snag if the cable is pulled over an edge or against a rough surface. During the splicing or accessing process, it can be tedious to remove the helical binders from long sections of the cable. To be effective, the binders must have a relatively short pitch (typically 1-3 cm). This means that the installer must make many cuts per meter of exposed cable (typically 1-3 m total). Also, to keep the binder threads from unwinding beyond the segments which are meant to be accessed, the end points of the segments to be accessed must be secured with tape or other materials. In addition, because the sub-units are normally stranded together with a continuous helix, they must be “unwound” to be separated from the cable. For cables having higher fiber counts (typically six or higher), two layers of sub-units are necessary, and each layer has its own binders, which can further complicate the accessing process.
Applicant has observed that known designs for bundled drop cable applications are not satisfactory for use in wiring MDUs. Those designs do not provide optical fibers of sufficient quantity or robustness to permit rapid wiring of the many residences or units within a large building. They also do not provide adequate means to access the optical fibers within the cable jacket, instead requiring tedious steps to perform a splicing and termination operation.
Applicant has found that an optical cable of many simplex sub-units that could be manufactured quickly and would be capable of being pulled and installed through an MDU in a single operation, while allowing individual sub-units of the cable to be readily separated for access and termination, would overcome the disadvantages observed in the art and fulfill a need in the industry.
For the purpose of the present description and of the appended claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term “about.” Also, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.